Image processing apparatus, image processing method, and storage medium

ABSTRACT

An image processing apparatus obtains a plurality of measured values obtained by measuring a measurement image; and replaces a first measured value with a value based on a second measured value. The first measured value being included in the plurality of measured values and being unsuitable for identifying the density characteristic, and the second measured value being included in the plurality of measured values and being suitable for identifying the density characteristic.

BACKGROUND OF THE DISCLOSURE Field of the Disclosure

The aspect of the embodiments relates to an image processing techniquefor obtaining density characteristics of each printing element to reducedensity unevenness and streaks which occur when ink is discharged toform an image.

Description of the Related Art

In printheads used for an inkjet printer, variations in the amount ofink discharge may occur among a plurality of printing elements (nozzles)due to a production error or the like. If variations in the amount ofink discharge occur, density unevenness is more likely to occur in aformed image. A head shading (HS) technique is a known technique forreducing density unevenness. In the HS technique, image data iscorrected based on information about the amount of ink discharge fromeach printing element (density characteristics of each printingelement). This correction processing makes it possible to increase ordecrease the number of ink dots to be discharged from each printingelement to adjust the density of an image to be formed.

To obtain density characteristics of each printing element, a method ofprinting each patch (e.g., a uniform image for each gradation) on paperand measuring the patch by a scanner is known. Japanese Patent Laid-OpenNo. 2013-237251 discusses a technique in which a recording medium ontowhich a pattern is fixed is set in a sheet feeding unit again and thepattern on the recording medium is scanned to obtain variationinformation and density information about the recording medium.

However, in the technique discussed in Japanese Patent Laid-Open No.2013-237251, the recording medium in the sheet feeding unit is setagain, which leads to an increase in the cost of obtaining densitycharacteristics.

SUMMARY OF THE DISCLOSURE

According to one aspect of the embodiments, an apparatus that corrects ameasured value obtained by measuring a measurement image formed by aprinting element configured to discharge ink, the measured value beingused for identifying a density characteristic of the printing element,the apparatus comprises: a first obtaining unit configured to obtain aplurality of measured values; and a first correction unit configured toreplace a first measured value with a value based on a second measuredvalue, the first measured value being included in the plurality ofmeasured values and being unsuitable for identifying the densitycharacteristic, the second measured value being included in theplurality of measured values and being suitable for identifying thedensity characteristic.

Further features of the disclosure will become apparent from thefollowing description of exemplary embodiments with reference to theattached drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

The accompanying drawings, which are incorporated in and constitute apart of the specification, illustrate embodiments of the disclosure and,together with the description, serve to explain the principles of thedisclosure.

FIG. 1 schematically illustrates a configuration of an image formingapparatus according to a first exemplary embodiment.

FIG. 2 is a block diagram illustrating a configuration of an imageforming system according to the first exemplary embodiment.

FIGS. 3A and 3B are block diagrams each illustrating a functionalconfiguration of an image processing apparatus according to the firstexemplary embodiment.

FIG. 4 is a flowchart illustrating head shading (HS) processing.

FIG. 5 illustrates an example of a measurement image.

FIGS. 6A and 6B are graphs each illustrating HS processing.

FIGS. 7A and 7B are graphs each illustrating an example of a measuredvalue obtained by measuring a measurement image.

FIG. 8 is a flowchart illustrating processing for correcting a measuredvalue.

FIGS. 9A and 9B are graphs each illustrating processing for correcting ameasured value.

FIG. 10 is a flowchart illustrating multi-color shading (MCS)processing.

FIG. 11 illustrates a three-dimensional color space for MCS processing.

FIGS. 12A and 12B are flowcharts each illustrating processing executedby the image processing apparatus.

FIG. 13 is a block diagram illustrating a functional configuration of anHS processing unit.

DESCRIPTION OF THE EMBODIMENTS

Exemplary embodiments of the disclosure will be described in detailbelow with reference to the accompanying drawings. The followingexemplary embodiments are not meant to limit the scope of the disclosureas encompassed by the appended claims. Not all features described in theexemplary embodiments are essential for the disclosure. The features maybe arbitrarily combined. Further, in the accompanying drawings, the sameor similar components are denoted by the same reference numerals, andredundant descriptions are omitted.

First Exemplary Embodiment

A first exemplary embodiment illustrates a method for obtaining densitycharacteristics of each printing element of an image forming apparatusin an image forming system including the image forming apparatus thatforms an image on a recording medium and a host apparatus that controlsthe image forming apparatus. In the image forming system, the densitycharacteristics of each printing element are obtained based on controlprocessing performed by the host apparatus. The first exemplaryembodiment illustrates an example where an inkjet printer is used as theimage forming apparatus and a personal computer (PC) is used as anexample of the host apparatus.

<Configuration of Image Forming Apparatus>

FIG. 1 schematically illustrates a configuration of an image formingapparatus according to the present exemplary embodiment. The imageforming apparatus according to the present exemplary embodiment is aninkjet printer. As illustrated in FIG. 1, an image forming apparatus 100includes printheads 101, 102, 103, and 104. Each of the printheads 101to 104 includes a plurality of nozzles for discharging ink. Theplurality of nozzles is arranged along a predetermined direction withina range corresponding to the width of a recording sheet 106. In otherwords, each of the printheads 101 to 104 according to the presentexemplary embodiment is a full-line printhead. The printhead 101 is aprinthead for discharging black (K) ink, and the printhead 102 is aprinthead for discharging cyan (C) ink. The printhead 103 is a printheadfor discharging magenta (M) ink, and the printhead 104 is a printheadfor discharging yellow (Y) ink. An interval between the nozzles in eachof the printheads 101 to 104 is set to have a resolution of 1200 dpi.

A conveyance roller 105 (and other rollers (not illustrated)) is rotatedby a driving force from a motor (not illustrated), thereby conveying therecording sheet 106, which is a recording medium, in a directionindicated by an arrow in FIG. 1. While the recording sheet 106 isconveyed, ink is discharged based on recording data from the pluralityof nozzles of each of the printheads 101 to 104. As a result, an imagecorresponding to one raster that corresponds to a nozzle row of each ofthe printheads 101 to 104 is sequentially formed. In addition, a scanner107 including scanning elements arranged at a predetermined pitch in astate where the scanning elements are arranged in parallel with theprintheads 101 to 104 is disposed at a position downstream of theprintheads 101 to 104 in a y-direction. The scanner 107 can scan imagesformed using the printheads 101 to 104 and can output the scanned imagesas multivalued data having a color signal value. For example, an imagecorresponding to one page can be formed by repeatedly performing an inkdischarge operation of discharging ink from each of the printheads 101to 104 on the conveyed recording sheet as described above.

A full-line type image forming apparatus is used as the image formingapparatus in the present exemplary embodiment. However, the presentexemplary embodiment can also be applied to a serial type image formingapparatus that performs recording by scanning each of the printheads 101to 104 in a direction crossing a recording sheet conveyance direction.

<Configuration of Image Forming System>

FIG. 2 is a block diagram illustrating the configuration of the imageforming system according to the present exemplary embodiment. Asillustrated in FIG. 2, the image forming system according to the presentexemplary embodiment includes the image forming apparatus 100illustrated in FIG. 1 and a PC 200 serving as the host apparatus thatcontrols the image forming apparatus 100.

The PC 200 includes a central processing unit (CPU) 201, a random accessmemory (RAM) 202, a hard disk drive (HDD) 203, a data transfer interface(I/F) 204, a keyboard/mouse I/F 205, and a display I/F 206. The CPU 201executes various processes based on programs held in the HDD 203 and theRAM 202. In particular, the CPU 201 executes programs to executeprocessing for an image processing apparatus 300 according to anexemplary embodiment to be described below. The RAM 202 is a volatilestorage and temporarily holds programs and data. The HDD 203 is anonvolatile storage and can hold programs and table data generated byprocessing according to each exemplary embodiment to be described below.The data transfer I/F 204 controls transmission and reception of data toand from the image forming apparatus 100. As a connection method fortransmitting and receiving data, a universal serial bus (USB), Instituteof Electrical and Electronics Engineers (IEEE) 1394, a local areanetwork (LAN), and the like can be used. The keyboard/mouse I/F 205 isan I/F for controlling a human interface device (HID) such as a keyboardor a mouse. A user inputs data via the keyboard/mouse I/F 205. Thedisplay I/F 206 controls display on a display (not illustrated).

The image forming apparatus 100 includes a CPU 211, a RAM 212, aread-only memory (ROM) 213, a data transfer I/F 214, a head controller215, an image processing accelerator 216, and a scanner controller 217.The CPU 211 executes processing based on programs held in the ROM 213and the RAM 212. The RAM 212 is a volatile storage and temporarily holdsprograms and data. The ROM 213 is a nonvolatile storage and holds dataand programs. The data transfer I/F 214 controls transmission andreception of data to and from the PC 200. The head controller 215supplies recording data to each of the printheads 101 to 104 illustratedin FIG. 1 and controls the printhead discharge operation. Specifically,the head controller 215 can be configured to read a control parameterand recording data from a predetermined address in the RAM 212. When theCPU 211 writes the control parameter and recording data at thepredetermined address in the RAM 212, processing is started by the headcontroller 215 and ink is discharged from each of the printheads 101 to104. The CPU 211 also functions as a formation control unit for forminga measurement image to be described below. The image processingaccelerator 216 executes image processing at a higher speed than the CPU211. Specifically, the image processing accelerator 216 can beconfigured to load a parameter and data for image processing from apredetermined address in the RAM 212. When the CPU 211 writes theparameter and data at the predetermined address in the RAM 212, theimage processing accelerator 216 is started to perform predeterminedimage processing. Note that the image processing accelerator 216 neednot necessarily be provided. The processing may be executed by the CPU211 alone depending on, for example, the specifications of the imageforming apparatus 100. The scanner controller 217 controls each scanningelement of the scanner 107 illustrated in FIG. 1 and outputs dataobtained by scanning to the CPU 211.

<Functional Configuration of Image Processing Apparatus>

FIG. 3A is a block diagram illustrating the functional configuration ofthe image processing apparatus 300 included in the PC 200. The imageprocessing apparatus 300 includes an input unit 301, an image processingunit 302, and an output unit 308.

As illustrated in FIG. 3A, the input unit 301 receives image data andoutputs the image data to the image processing unit 302. The imageprocessing unit 302 includes an input color conversion processing unit303, an ink color conversion processing unit 304, and a head shading(HS) processing unit 305. The image processing unit 302 further includesa tone reproduction curve (TRC) processing unit 306 and a quantizationprocessing unit 307.

The input color conversion processing unit 303 converts the input imagedata obtained from the input unit 301 into image data corresponding to acolor reproduction range of a printer. The input image data used in thepresent exemplary embodiment is data representing coordinates (R, G, B)in a standard red, green, and blue (sRGB) color space, which is a colorspace corresponding to a display. The sRGB color space is a space inwhich “R”, “G”, and “B” are each set as an axis, and each coordinate isrepresented by eight bits. Accordingly, the input image data is imagedata in which R, G, and B values are each represented by eight bits. Theinput color conversion processing unit 303 converts an input colorsignal value for each of the R, G, and B values in the input image datainto a printer color signal value for each of R′, G′, and B′ valuescorresponding to the color reproduction range of the printer. The colorsignal value for each of the R, G, and B values is hereinafter expressedas a color signal value (R, G, B). In the conversion processing, a knownmethod, such as matrix operation processing or processing using athree-dimensional lookup table (LUT), is used. In the present exemplaryembodiment, the conversion processing is performed using thethree-dimensional LUT and an interpolation operation. The resolution of8-bit image data that is treated in the image processing unit 302 is1200 dpi.

The ink color conversion processing unit 304 performs conversionprocessing for converting the color signal values in the image dataconverted by the input color conversion processing unit 303 into colorsignal values corresponding to a plurality of types of ink. The imageforming apparatus 100 uses black (K) ink, cyan (C) ink, magenta (M) ink,and yellow (Y) ink. Accordingly, a printer color signal value (R′, G′,B′) is converted into an ink color signal value (K, C, M, Y). The K, C,M, and Y values are also represented by eight bits, like the R, G, and Bvalues. Like the input color conversion processing unit 303, the inkcolor conversion processing unit 304 also performs the conversionprocessing using the three-dimensional LUT and the interpolationoperation.

The HS processing unit 305 performs correction processing based on thedensity characteristics of the nozzles constituting each of theprintheads 101 to 104 on the image data having the ink color signalvalue (K, C, M, Y). FIG. 13 is a block diagram illustrating the detailedfunctional configuration of the HS processing unit 305. The HSprocessing unit 305 includes an image data obtaining unit 1301, ameasured value obtaining unit 1302, a measured value correction unit1303, a target obtaining unit 1307, and a color signal value correctionunit 1308. The measured value correction unit 1303 includes a normalvalue obtaining unit 1304, a representative value calculation unit 1305,and a replacement processing unit 1306. HS processing performed by theHS processing unit 305 will be described in detail below.

The TRC processing unit 306 adjusts, for each color of ink, the numberof ink dots to be recorded by the image forming apparatus 100 on imagedata having an HS color signal value (K′, C′, M′, Y′) obtained by the HSprocessing. Specifically, the image data is corrected such that therelationship between the number of dots to be recorded on a recordingmedium and the brightness achieved by the dots becomes linear. Thiscorrection processing enables adjustment of the number of dots to berecorded on a recording medium.

The quantization processing unit 307 performs quantization processing(halftone processing) on image data having a TRC color signal value (K″,C″, M″, Y″) obtained by TRC processing, and generates binary data inwhich each pixel value is represented by one bit. The binary data thatis used as recording data represents an arrangement of ink dots to bedischarged. In the present exemplary embodiment, a known ditheringmethod is used as a method for quantization processing, but instead, aknown error diffusion method or the like may be used.

The output unit 308 outputs the binary data obtained by the quantizationprocessing to the image forming apparatus 100. The image formingapparatus 100 drives each of the printheads 101 to 104 based on thereceived binary data and discharges ink of each color onto a recordingmedium to form an image on the recording medium.

<Processing Executed by Image Processing Apparatus>

FIG. 12A is a flowchart illustrating processing executed by the imageprocessing apparatus 300. The processing executed by the imageprocessing apparatus 300 will be described in detail below withreference to FIG. 12A. In the following description, each step isdenoted by “S” followed by a number.

In step S1201, the input unit 301 receives input image data and outputsthe received image data to the image processing unit 302. In step S1202,the input color conversion processing unit 303 converts the input colorsignal value (R, G, B) of the input image data into the printer colorsignal value (R′, G′, B′) corresponding to the color reproduction rangeof the printer. In step S1203, the ink color conversion processing unit304 coverts the printer color signal value (R′, G′, B′) into the inkcolor signal value (K, C, M, Y) corresponding to a plurality of types ofink. In step S1204, the HS processing unit 305 performs HS processing onthe image data having the ink color signal value (K, C, M, Y). In stepS1205, the TRC processing unit 306 performs TRC processing on the imagedata having the HS color signal value (K′, C′, M′, Y′) obtained by theHS processing. In step S1206, the quantization processing unit 307performs quantization processing on the image data having the TRC colorsignal value (K″, C″, M″, Y″) obtained by the TRC processing. In stepS1207, the output unit 308 outputs the binary data generated by thequantization processing to the image forming apparatus 100.

<HS Processing>

FIG. 4 is a flowchart illustrating the HS processing performed by the HSprocessing unit 305. The HS processing will be described in detail belowwith reference to FIG. 4.

In step S401, the image data obtaining unit 1301 obtains image datahaving the ink color signal value (K, C, M, Y) output from the ink colorconversion processing unit 304. In step S402, the measured valueobtaining unit 1302 obtains a measured value for identifying densitycharacteristics of each nozzle. The measured value is obtained as imagedata in advance by preliminarily measuring the measurement image by thescanner 107, and the image data is held in the HDD 203 or the like.

A method for generating image data including a measured value will bedescribed below. First, a measurement image for obtaining densitycharacteristics of each nozzle is formed on the recording sheet 106.FIG. 5 illustrates an example of the measurement image. Patchescorresponding to nine gradations of 501, 502, 503, 504, 505, 506, 507,508, and 509, respectively, are formed on the recording sheet 106. Eachpatch is formed using only ink of a single color. An example where apatch is formed using only the printhead 101 (K ink) will be describedbelow. In the present exemplary embodiment, processing for forming themeasurement image uses only ink of a single color, and thus theprocessing is performed through a bypass path 309 indicated by a dashedline in FIG. 3A. With this configuration, the input image data can bedirectly input to the TRC processing unit 306 without passing throughthe input color conversion processing unit 303, the ink color conversionprocessing unit 304, and the HS processing unit 305.

Next, the measurement image is scanned by the scanner 107 and a scannedimage is obtained by scanning Each pixel value of the scanned image isobtained using three channels of (R, G, B). Next, the scanned image isconverted into a scanned image in which a pixel value corresponding toone channel is included in each pixel by using a color conversion table,which is prepared in advance, according to the color characteristics ofthe scanner 107. In the present exemplary embodiment, the scanned imageis converted into a 16-bit value that is linear to “Y” in a CIE XYZcolor space. Any color space can be used to represent each pixel valueof the scanned image obtained after color conversion. For example, “L*”in a CIEL*a*b* color space, or a density may be used. In a case wherethe measurement image is formed using color ink of C, M, Y, or the like,a value corresponding to chroma can also be used instead of a valuecorresponding to brightness. For example, R, G, and B values may be usedas values corresponding to complementary colors of C, M, and Y,respectively. The resolution of the scanned image in the presentexemplary embodiment is 1200 dpi. The above-described processing makesit possible to generate image data having each pixel value of thescanned image as the measured value and to obtain the image data in stepS402.

In step S403, the measured value correction unit 1303 corrects themeasured value obtained in step S402. Processing for correcting themeasured value will be described in detail below. In step S404, thetarget obtaining unit 1307 obtains target characteristic datarepresenting a target characteristic based on a measurement curvegenerated based on the corrected measured value. The term “targetcharacteristic” used herein refers to a predetermined target densitycharacteristic based on a measurement curve for each nozzle. Asillustrated in FIG. 6A, a horizontal axis represents a gradation and avertical axis represents a measured value. Referring to FIG. 6A, astraight line that is linear to a gradation represents a targetcharacteristic 604. A dashed line 601 indicates an upper limit of thehorizontal axis. Since the input signal value is represented by an 8-bitvalue, the upper limit in the present exemplary embodiment is 255. Ameasurement curve 602 is a curve generated by plotting the measuredvalue in the scanned image of each of the patches 501 to 509 andperforming an interpolation operation. A known piecewise linearinterpolation is used for the interpolation operation in the presentexemplary embodiment. A known spline curve or the like may be used asthe measurement curve. The measurement curve 602 represents the densitycharacteristic of a nozzle corresponding to a pixel position “x”, and anumber of curves corresponding to the number of nozzles used for formingthe measurement image are obtained. Different measurement curves for thedensity characteristics of each nozzle are obtained. For example, themeasurement curve for the nozzle that discharges a small amount of inkis shifted upward (in a direction in which the brightness increases). Ineach gradation, a number of measured values corresponding to the widthof each patch in the y-direction are obtained. Accordingly, averagevalues of the measured values obtained in the y-direction in eachgradation are used as nine measured values to generate the measurementcurve.

In step S405, the color signal value correction unit 1308 corrects theink color signal value (K, C, M, Y) of the image data obtained in stepS401 based on the measured value calculated in step S403, and obtainsthe HS color signal value (K′, C′, M′, Y′). A method for obtaining theHS color signal value (K′, C′, M′, Y′) will be described with referenceto FIG. 6B. Referring to FIG. 6B, an input value 605 represents the inkcolor signal value (K, C, M, Y). The color signal value correction unit1308 obtains the value of the target characteristic 604 corresponding tothe input value 605, and uses the obtained value as a target value 606.Further, a signal value 607 of a measurement curve 603 corresponding tothe target value 606 is obtained as the corrected HS color signal value(K′, C′, M′, Y′). The measurement curve 603 used herein is a measurementcurve obtained by performing a piecewise linear interpolation on themeasured value corrected in step S403.

<Processing for Correcting Measured Value>

Processing for correcting a measured value will be described in detailbelow. FIGS. 7A and 7B are graphs each illustrating an example of ameasured value for the measurement image illustrated in FIG. 5. In thegraphs illustrated in FIGS. 7A and 7B, a horizontal axis represents anozzle number, which corresponds to the number of nozzles included inthe printhead 101, and a vertical axis represents a measured value. Ameasured value 701 is a measured value in a region corresponding to thepatch 509 illustrated in FIG. 5. Similarly, measured values in regionscorresponding to the patches 501 to 508, respectively, are alsoobtained, but the illustration of the measured values is omitted tosimplify the explanation. As described above, a number of measuredvalues corresponding to the width of each patch in the y-direction ineach gradation are obtained. In this case, however, the average value ofthe measured values in the y-direction in the gradation of the patch 509is illustrated.

FIG. 7A is a graph illustrating the measured value when no distortionoccurs in the recording sheet on which the measurement image is formed.This is an example of the measured value obtained in the configurationof the apparatus in which the printheads 101 to 104 and the scanner 107illustrated in FIG. 1 are installed at a sufficiently small distance. Onthe other hand, FIG. 7B illustrates the measured value when distortionoccurs in the recording sheet on which the measurement image is formed.This is an example of the measured value obtained in the configurationof the apparatus in which the printheads 101 to 104 and the scanner 107are installed at a large distance, or a drying step is provided betweenthe printheads 101 to 104 and the scanner 107. This configuration of theapparatus enables a highly accurate measurement by performing themeasurement after the color of each ink recorded on the recording sheetis stabilized.

Comparing FIG. 7B with FIG. 7A, it is obvious that the measured value atan end of the recording sheet cannot be obtained due to the distortionof the recording sheet. A measured value 702 is a measured value at acentral portion in an x-direction of the recording sheet, and representsa characteristic similar to that represented by the measured value 701when no distortion occurs. Measured values 703 and 704 are measuredvalues obtained at ends in the x-direction of the recording sheet, andthe interval of waveforms of the measured values 703 and 704 is smallerthan that of the measured value 702 obtained at the central portion.This is because the ends of the recording sheet are uplifted and therecording sheet is inclined with respect to the scanner 107. Measuredvalues 705 and 706 are measured values obtained at extreme ends in thex-direction of the recording sheet, and include a measured valuecorresponding to a blank region in which the patches illustrated in FIG.5 are not formed. Accordingly, the HS processing unit 305 according tothe present exemplary embodiment replaces a measured value correspondingto a region in which the measured value cannot be accurately obtained(the region is hereinafter referred to as an abnormal region) with ameasured value corresponding to a region in which the measured value canbe accurately obtained (the region is hereinafter referred to as anormal region). The normal region in the present exemplary embodiment isa region corresponding to a central portion of the recording sheet thatis not affected by the distortion of the recording sheet. The abnormalregion in the present exemplary embodiment is a region corresponding toeach end and each extreme end of the recording sheet which are affectedby the distortion of the recording sheet. It is assumed that the normalregion and the abnormal region are set in advance.

FIG. 8 is a flowchart illustrating processing for correcting a measuredvalue in the HS processing unit 305. In step S801, the normal valueobtaining unit 1304 obtains the measured value corresponding to thenormal region. Among the measured values illustrated in FIG. 9A, themeasured value 702 corresponding to the central portion indicated by adashed line is obtained as the measured value corresponding to thenormal region. In the present exemplary embodiment, 320 nozzlescorrespond to the central portion. Accordingly, the normal valueobtaining unit 1304 obtains 320 measured values as the measured value702.

In step S802, the representative value calculation unit 1305 calculatesa representative value of the measured values corresponding to thenormal region. In the present exemplary embodiment, density unevennessis extracted every 32 nozzles. Therefore, in a case where numbers 1 to320 are added to the 320 obtained measured values, respectively, a firstrepresentative value is obtained by averaging 10 measured values withthe numbers of 1, 33, 65, 97, 129, 161, 193, 225, 257, and 289,respectively. Similarly, second and third representative values arecalculated to thereby obtain 32 representative values 901.

In step S803, the replacement processing unit 1306 corrects the measuredvalue corresponding to the abnormal region. Specifically, thereplacement processing unit 1306 replaces the measured valuecorresponding to the abnormal region with the representative valuecalculated in step S802 as illustrated in FIG. 9B. For example, in acase where 320 measured values are obtained as the measured value 703,the measured value 703 is repeatedly replaced with the 320 replacedmeasured values 903 generated by repeatedly replacing the 32representative values 901. Similarly, replaced measured values 904, 905,and 906 are obtained by repeatedly replacing the 32 representativevalues 901. Since the measured values are directly replaced with the 32representative values 901, it may be desirable to set the size of theabnormal region to be an integral multiple of the size of the normalregion. Since the replacement processing is not performed on themeasured value corresponding to the normal region, a measured value 902is the same as the measured value 702.

Advantageous Effects of First Exemplary Embodiment

As described above, the image processing apparatus 300 according to thepresent exemplary embodiment obtains a plurality of measured values bymeasuring a measurement image. A measured value that is included in theplurality of obtained measured values and is unsuitable for identifyingthe density characteristics of each printing element is replaced with avalue that is suitable for identifying the density characteristicscorresponding to a measured value. With this configuration, even when ameasured value that cannot be used is included in the measured valuesobtained by measuring the measurement image, the density characteristicsof each printing element can be obtained with high accuracy. Inaddition, the HS processing using the density characteristics of eachprinting element can reduce density unevenness and streaks in an imageformed on a recording medium.

Modified Examples

In step S802 according to the exemplary embodiment described above, theaverage value of measured values is used as the representative value.However, the representative value is not limited to the average value.For example, a median may be used as the representative value.Alternatively, the average value may be calculated after weighting isperformed based on the position of each nozzle. In the case ofcalculating the representative value, the measured value in a regioncorresponding to a non-discharge nozzle can be excluded from being used.

In the exemplary embodiment described above, variations in the measuredvalues are extracted every 32 nozzles, but instead may be extractedevery eight nozzles, or every 64 nozzles. In addition, the number ofnozzles for which variations in the measurement value are extracted canbe determined by estimating a cycle by a method, such as a frequencyanalysis or correlation calculation, on the measured value correspondingto the normal region.

In the exemplary embodiment described above, the scanning resolution forthe measurement image is 1200 dpi, which is equal to the resolution forthe arrangement of the nozzles of each of the printheads 101 to 104.However, the scanning resolution may be higher or lower than theresolution for the arrangement of the nozzles of each of the printheads101 to 104. If the scanning resolution is increased, the density foreach nozzle can be obtained more accurately. On the other hand, if thescanning resolution is decreased, it is difficult to detecthigh-frequency unevenness. In this case, however, the amount of data tobe read decreases, which leads to a reduction in the cost of the system.Further, the measurement curve may be generated using the average valueof measured values for a plurality of nozzles. This leads to a reductionin storage capacity for holding information about the measurement curve.

In the exemplary embodiment described above, each end of the recordingsheet that is greatly affected by distortion of the recording sheet isset as the abnormal region. However, the abnormal region is alsogenerated due to a factor other than distortion of the recording sheet.For example, a local region in which a non-discharge nozzle, stain on arecording sheet, a flaw on a surface of an image, or the like is presentmay be set as the abnormal region. In this case, the image formingsystem may include a detection unit that detects a non-discharge nozzle,stain on a recording sheet, or the like.

In a case where a recording sheet, such as embossed paper, which has apattern formed on the entire surface thereof and on which HS processingcannot be performed, is used, the entire region of the recording sheetcan be regarded as the abnormal region. In this case, the measured valuein the entire region of the recording sheet is replaced with therepresentative value. If a disturbance caused by the pattern on therecording sheet is isotropic, the representative value calculated byaveraging can be regarded as the value on which noise reductionprocessing has been performed. This may lead to an improvement in theaccuracy of HS processing.

While the exemplary embodiment described above illustrates an examplewhere the measured value is corrected by replacement processing everytime an image is formed on a recording medium, the measured valuecorresponding to the abnormal region may be replaced with the measuredvalue corresponding to the normal region in advance. In this case, theprocesses of steps S402 and S403 are performed in advance by themeasured value obtaining unit 1302 and the measured value correctionunit 1303, respectively, and the corrected measured value is held in theHDD 203 or the like. This eliminates the performing correctionprocessing every time image data is input, which leads to a reduction inprocessing cost and a reduction in density unevenness in an image.

Second Exemplary Embodiment

In the exemplary embodiment described above, each patch of themeasurement image is formed using only ink of a single color and HSprocessing is performed on each color of ink. However, even after the HSprocessing is performed on each color of ink, color unevenness may occurwhen a multi-order color is represented by superimposing two or morecolors of ink. To deal with such color unevenness, a technique calledmulti-color shading (MCS) processing is known. Accordingly, a secondexemplary embodiment illustrates processing for reducing densityunevenness and streaks in an image with high accuracy even when ameasured value that cannot be used is included in the measured valuesobtained for MCS processing.

<Functional Configuration of Image Processing Apparatus>

FIG. 3B is a block diagram illustrating the functional configuration ofthe image processing apparatus 300 according to the second exemplaryembodiment. Image data output from the input color conversion processingunit 303 is input to an MCS processing unit 310. The MCS processing unit310 performs correction processing based on the density characteristicsof the nozzles constituting each of the printheads 101 to 104 on theprinter color signal value (R′, G′, B′) in the input image data. The MCSprocessing will be described in detail below. The image data having acorrected MCS color signal value (R″, G″, B″) is output to the ink colorconversion processing unit 304. Like the HS processing unit 305, the MCSprocessing unit 310 obtains the measured value by measuring each patchin advance. The MCS processing unit 310 has a functional configurationsimilar to the functional configuration of the HS processing unit 305illustrated in FIG. 13.

A measurement image for MCS processing includes a plurality of patchesobtained by changing the input signal values R, G, and B independently.In the present exemplary embodiment, five gradations of 0, 64, 128, 192,and 25 are set for each of the R, G, and B values and 53 combinations(=125) of multi-order color patches are formed. The combinations of thepatches are not limited to this example. In the present exemplaryembodiment, processing for forming the measurement image for MCSprocessing is performed through a bypass path 311 indicated by a dashedline in FIG. 3B. Thus, the image obtained after correction processing isperformed by the HS processing unit 305 can be used as the measurementimage. The measurement image is scanned by the scanner 107 and a scannedimage is obtained. Unlike in the first exemplary embodiment, each pixelvalue of the scanned image is not converted into a value correspondingto one channel, but instead is held in the HDD 203 as an image havingpixel values each corresponding to three channels of (R, G, B).

<Processing Executed by Image Processing Apparatus>

FIG. 12B is a flowchart illustrating processing executed by the imageprocessing apparatus 300. The processing executed by the imageprocessing apparatus 300 will be described in detail below withreference to FIG. 12B.

In step S1211, the input unit 301 receives input image data and outputsthe received image data to the image processing unit 302. In step S1212,the input color conversion processing unit 303 converts the input colorsignal value (R, G, B) in the input image data into the printer colorsignal value (R′, G′, B′) corresponding to the color reproduction rangeof the printer. In step S1213, the MCS processing unit 310 performs MCSprocessing on the image data having the printer color signal value (R′,G′, B′). In step S1214, the ink color conversion processing unit 304converts the MCS color signal value (R″, G″, B″) into the ink colorsignal value (K, C, M, Y) corresponding to a plurality of types of ink.In step S1215, the HS processing unit 305 performs HS processing on theimage data having the ink color signal value (K, C, M, Y). In stepS1216, the TRC processing unit 306 performs TRC processing on the imagedata having the HS color signal value (K′, C′, M′, Y′) obtained by theHS processing. In step S1217, the quantization processing unit 307performs quantization processing on the image data having the TRC colorsignal value (K″, C″, M″, Y″) obtained by the TRC processing. In stepS1218, the output unit 308 outputs the binary data generated by thequantization processing to the image forming apparatus 100.

<MCS Processing>

The MCS processing will be described below with reference to a flowchartillustrated in FIG. 10. The MCS processing mainly differs from the HSprocessing in that the measurement image illustrated in FIG. 5 is amulti-order color image and the measured value corresponds to threechannels of (R, G, B).

In step S1001, the MCS processing unit 313 obtains image data having theprinter color signal value (R′, G′, B′) output from the input colorconversion processing unit 303. In step S1002, the MCS processing unit313 obtains, from the scanned image, the measured value at a nozzleposition corresponding to a pixel of interest. In the present exemplaryembodiment, 125 color signal values (R, G, B) are obtained as measuredvalues for 125 patches. The measured values are obtained as image databy measuring the measurement image in advance by the scanner 107, andthe image data is held in the HDD 203 or the like.

In step S1003, the MCS processing unit 313 corrects the measured value.The correction processing is performed by the processing illustrated inFIG. 8 in the same manner as in the first exemplary embodiment. Thesecond exemplary embodiment differs from the first exemplary embodimentin that the representative value in the normal region is determined foreach of the three channels of (R, G, B) and the measured valuecorresponding to the abnormal region is replaced with the representativevalue. In step S1004, the MCS processing unit 313 obtains the targetvalue (R, G, B). In the present exemplary embodiment, the target value(R, G, B) is obtained with reference to the LUT (not illustrated) inwhich the correspondence relation between the printer color signal value(R′, G′, B′) in the image data obtained in step S1001 and the targetvalue (R, G, B) of the scanned image is held.

In step S1005, the printer color signal value (R′, G′, B′) in the imagedata obtained in step S1001 is corrected based on the corrected measuredvalue, and the MCS color signal value (R″, G″, B″) is obtained. Aspecific method for correction processing will be described withreference to FIG. 11. FIG. 11 illustrates a three-dimensional colorspace having “R”, “G”, and “B” of the color signal value (R, G, B) inthe scanned image as each axis. A target value 1101 represents thetarget value obtained in step S1004. Values 1102, 1103, 1104, and 1105are four corrected measured values selected to form a minimumtetrahedron including the target value 1001 from among the 125 correctedmeasured values obtained in step S1003. The distance between the targetvalue 1101 and each of the four corrected measured values 1102 to 1105is calculated and an interpolation operation using the four correctedmeasured values is performed based on the ratio of the distance. Thevalue obtained by the interpolation operation is obtained as the MCScolor signal value (R″, G″, B″) corresponding to the pixel of interest.

Advantageous Effects of Second Exemplary Embodiment

As described above, the image processing apparatus 300 according to thesecond exemplary embodiment replaces the measured value corresponding tothe abnormal region with the measured value corresponding to the normalregion and corrects the color signal value of the image by MCSprocessing using the replaced measured value. With this configuration,even when a measured value that cannot be used is included in themeasured values obtained by measuring the measurement image, the densitycharacteristics of each printing element can be obtained with highaccuracy. In addition, the MCS processing using the densitycharacteristics of each printing element makes it possible to reducecolor unevenness in an image formed on a recording medium.

Modified Examples

The exemplary embodiment described above illustrates an example wherethe PC 200 functions as the image processing apparatus 300 in accordancewith software installed in the PC 200. Alternatively, the image formingapparatus 100 may include the image processing apparatus 300. In a casewhere the image processing apparatus 300 is mounted in the image formingapparatus 100, a dedicated image processing circuit that can achieveeach functional configuration of the image processing apparatus 300 maybe implemented. The functions of the image processing apparatus 300 maybe executed by a server that can communicate with the image formingapparatus 100. More alternatively, a part of the image processingapparatus 300 may be configured in the PC 200, and the other part of theimage processing apparatus 300 may be configured in the image formingapparatus 100.

In the exemplary embodiments described above, an RGB color space is usedas the color space representing each signal value of the measurementimage, but any color space can be used. For example, a CIE XYZ colorspace or a CIEL*a*b* color space may be used.

In the exemplary embodiments described above, an image is formed usingink of four colors of K, C, M, and Y, but an image may be formed usingother types of ink. For example, the above-described exemplaryembodiments can also be applied to an image forming apparatus that formsan image using low-density ink of colors, such as light cyan, lightmagenta, and gray, or using ink of specific colors, such as red, green,blue, orange, and violet.

The exemplary embodiments described above illustrate an example wherethe image processing unit 302 performs processing on image data input inan RGB format represented by color signal values corresponding to threeprimary colors, but image data in a KCMY format may be directly input tothe image processing unit 302. In this case, the processing performed bythe input color conversion processing unit 303 and the processingperformed by the ink color conversion processing unit 304 can be omittedin the image processing unit 302.

Other Embodiments

Embodiment(s) of the disclosure can also be realized by a computer of asystem or apparatus that reads out and executes computer executableinstructions (e.g., one or more programs) recorded on a storage medium(which may also be referred to more fully as a ‘non-transitorycomputer-readable storage medium’) to perform the functions of one ormore of the above-described embodiment(s) and/or that includes one ormore circuits (e.g., application specific integrated circuit (ASIC)) forperforming the functions of one or more of the above-describedembodiment(s), and by a method performed by the computer of the systemor apparatus by, for example, reading out and executing the computerexecutable instructions from the storage medium to perform the functionsof one or more of the above-described embodiment(s) and/or controllingthe one or more circuits to perform the functions of one or more of theabove-described embodiment(s). The computer may comprise one or moreprocessors (e.g., central processing unit (CPU), micro processing unit(MPU)) and may include a network of separate computers or separateprocessors to read out and execute the computer executable instructions.The computer executable instructions may be provided to the computer,for example, from a network or the storage medium. The storage mediummay include, for example, one or more of a hard disk, a random-accessmemory (RAM), a read only memory (ROM), a storage of distributedcomputing systems, an optical disk (such as a compact disc (CD), digitalversatile disc (DVD), or Blu-ray Disc (BD)™), a flash memory device, amemory card, and the like.

While the disclosure has been described with reference to exemplaryembodiments, it is to be understood that the disclosure is not limitedto the disclosed exemplary embodiments. The scope of the followingclaims is to be accorded the broadest interpretation so as to encompassall such modifications and equivalent structures and functions.

This application claims the benefit of Japanese Patent Application No.2019-180963, filed Sep. 30, 2019, which is hereby incorporated byreference herein in its entirety.

What is claimed is:
 1. An apparatus that corrects a measured valueobtained by measuring a measurement image formed by a printing elementconfigured to discharge ink, the measured value being used foridentifying a density characteristic of the printing element, theapparatus comprising: a first obtaining unit configured to obtain aplurality of measured values; and a first correction unit configured toreplace a first measured value with a value based on a second measuredvalue, the first measured value being included in the plurality ofmeasured values and being unsuitable for identifying the densitycharacteristic, the second measured value being included in theplurality of measured values and being suitable for identifying thedensity characteristic.
 2. The apparatus according to claim 1, whereinthe first measured value is a measured value corresponding to a regionin which distortion of a recording medium occurs in the measurementimage formed on the recording medium.
 3. The apparatus according toclaim 2, wherein the distortion of the recording medium occurs at an endof the recording medium.
 4. The apparatus according to claim 3, whereinthe distortion of the recording medium occurs such that the end of therecording medium is uplifted.
 5. The apparatus according to claim 2,wherein the first measured value is a measured value corresponding to aregion with an inclination with respect to a scanner in a region of themeasurement image, the scanner being configured to measure themeasurement image.
 6. The apparatus according to claim 1, wherein thefirst measured value is at least one of a measured value correspondingto a printing element configured not to discharge ink, a measured valueobtained by measuring a flaw on a surface of the measurement image, anda measured value obtained by measuring a region corresponding to dirt ona surface of the recording medium on which the measurement image isformed.
 7. The apparatus according to claim 1, wherein the firstmeasured value is a measured value obtained by measuring a predeterminedfirst region in the measurement image, and wherein the second measuredvalue is a measured value obtained by measuring a predetermined secondregion in the measurement image.
 8. The apparatus according to claim 1,wherein the first correction unit replaces a plurality of the firstmeasured values with a plurality of representative values based on aplurality of the second measured values.
 9. The apparatus according toclaim 1, wherein the first correction unit replaces the measured valuesfor each gradation of the measurement image.
 10. The apparatus accordingto claim 1, wherein the first correction unit replaces the measuredvalues for each color of ink for an image forming apparatus including aplurality of types of ink.
 11. The apparatus according to claim 1,wherein the first correction unit replaces the measured values for eachcombination of colors of ink for an image forming apparatus including aplurality of types of ink.
 12. The apparatus according to claim 1,further comprising: a second obtaining unit configured to obtain imagedata representing an image to be formed on a recording medium; and asecond correction unit configured to correct a signal value in the imagedata based on the plurality of measured values, at least some of theplurality of measured values being replaced with a value based on thesecond measured value.
 13. The apparatus according to claim 12, furthercomprising a third obtaining unit configured to obtain targetcharacteristic data representing a target density characteristic of theprinting element, wherein the second correction unit corrects the signalvalue in the image data based on the target density characteristic andthe density characteristic of the printing element specified by theplurality of measured values, at least some of the plurality of measuredvalues being replaced with a value based on the second measured value.14. The apparatus according to claim 13, wherein the second correctionunit uses, as the density characteristic of the printing element, acurve generated by an interpolation operation on the plurality ofmeasured values, at least some of the plurality of measured values beingreplaced with a value based on the second measured value.
 15. Anon-transitory computer-readable storage medium storing a program forcausing a computer to perform a method for correcting a measured valueobtained by measuring a measurement image formed by a printing elementconfigured to discharge ink, the measured value being used foridentifying a density characteristic of the printing element, the methodcomprising: obtaining a plurality of measured values; and replacing afirst measured value with a value based on a second measured value, thefirst measured value being included in the plurality of measured valuesand being unsuitable for identifying the density characteristic, thesecond measured value being included in the plurality of measured valuesand being suitable for identifying the density characteristic.
 16. Amethod for correcting a measured value obtained by measuring ameasurement image formed by a printing element configured to dischargeink, the measured value being used for identifying a densitycharacteristic of the printing element, the image processing methodcomprising: obtaining a plurality of measured values; and replacing afirst measured value with a value based on a second measured value, thefirst measured value being included in the plurality of measured valuesand being unsuitable for identifying the density characteristic, thesecond measured value being included in the plurality of measured valuesand being suitable for identifying the density characteristic.
 17. Themethod according to claim 16, wherein the first measured value is ameasured value corresponding to a region in which distortion of arecording medium occurs in the measurement image formed on the recordingmedium.
 18. The method according to claim 17, wherein the distortion ofthe recording medium occurs at an end of the recording medium.
 19. Themethod according to claim 18, wherein the distortion of the recordingmedium occurs such that the end of the recording medium is uplifted. 20.The method according to claim 17, wherein the first measured value is ameasured value corresponding to a region with an inclination withrespect to a scanner in a region of the measurement image, the scannerbeing configured to measure the measurement image.